Reflection-type liquid crystal device with polarization of output light perpendicular to that of input light

ABSTRACT

A reflection-type liquid crystal (LC) device having a twisted nematic (TN) LC layer that lets linearly-polarized incident light enter and become circularly-polarized at a reflecting surface, and then linearly polarizes it, after reflecting, with a plane of polarization that has been rotated 90° from the incident light at an light output surface. The TN LC layer allows linearly-polarized incident light to enter at an angle to the molecular plane of light input of the twisted nematic LC, and to linearly polarize the light after reflection. The plane of polarization is rotated 90° from the incident light at the light output surface. The TN LC layer also allows circularly-polarized incident light to enter and become linearly-polarized light at the reflecting surface. Circularly-polarized light after reflection may be rotated opposite the incident circularly-polarized light at the light output surface to provide for reverse on/off states.

This is a continuation of copending application Ser. No. 07/739,961filed Aug. 5, 1991, now abandoned.

BACKGROUND OF THE PRESENT INVENTION

1. Field of the Present Invention

The present invention relates generally to liquid crystal devices andspecifically to reflection-type devices that have reduced double imagereflections and improved performance.

2. Description of the Prior Art

Liquid crystal (LC) has the unusual physical characteristic of sharingaspects of both the liquid and solid phases at the same time. Computerand television display screens that are based on LC technology are nowwidely available. LC displays (LCDs) are typically flat, draw littlepower, have good contrast, and can be made to produce color.

Prior art reflection-type LCDs use twisted nematic (TN) liquid crystalmaterial with an optically uniaxial opto-electric medium having aquarter wavelength plate with a twist angle of 45°. The output light isvery often elliptically-polarized. For a description of such devices,see, U.S. Pat. No. 4,019,807, and Japanese Laid-Open Patent Publication56-43681. Some prior art reflection-type LCDs use super twisted nematic(STN) mode, which is a variation on the TN mode mentioned.

In prior art reflection-type LCDs, there has been little allowablemargin with respect to the thickness of an LC layer, and displayperformance has been inconsistent. Also, light output is reduced becauseit is typically elliptically-polarized. Dark and discolored displays anddouble images are significant problems in prior art LCDs. A liquidcrystal device that solves these problems is, therefore, needed forpresent and future applications.

SUMMARY OF THE PRESENT INVENTION

An object of the present invention is to produce a reflection-type LCdevice that reduces light loss, has increased production margins, andeliminates the discoloration and double imaging problems inherent inprior art displays.

An embodiment of the present invention is a reflection-type LC devicehaving a TN LC layer that lets linearly-polarized incident light enterand become circularly-polarized at a reflecting surface, and thenlinearly polarizes it, after reflecting, with a plane of polarizationthat has been rotated 90° from the incident light at an light outputsurface. The twisted nematic LC layer allows linearly-polarized incidentlight to enter at an angle to the molecular plane of light input of thetwisted nematic LC, and to linearly polarize the light after reflection.The plane of polarization is rotated 90° from the incident light at thelight output surface. The twisted nematic LC layer also allowscircularly-polarized incident light to enter and becomelinearly-polarized light at the reflecting surface. Circularly-polarizedlight after reflection which has been rotated opposite the incidentcircularly-polarized light at the light output surface.

An advantage of the present invention is that one polarizing platecombines polarizing, analyzer, and reflecting elements.

Another advantage of the present invention is that relative roughnessthe surface facing the LC device of at least one of the substrates ofthe two substrates of the LC cell.

Another advantage of the present invention is that a reflecting elementis on a surface facing the LC side of the LC cell substrate.

Another advantage of the present invention is that it has an Δnd valuerelatively larger than that typically found in the prior art. This makespossible wider production tolerances, with respect to how thick the LClayer must be.

Another advantage of the present invention is that a linearly-polarizedlight output results in reduced light losses and more visible displays.

Another advantage of the present invention is that it facilitatescontrol of the opto-electric characteristics, so that the multiplexingperformance is improved in devices with a steep characteristic andgray-scale expression is improved in devices with a moderatecharacteristic.

Another advantage of the present invention is that it flattens thereflection spectrum when OFF, which reduces light loss over a wide rangeof wavelengths and prevents discoloration.

Another advantage of the present invention is that there is no reductionof the numerical aperture depending on the means by which the pictureelements are addressed, and so light loss is further reduced.

Another advantage of the present invention is that an LC display elementcan be offered which eliminates double images in the display.

Another advantage of the present invention is that by using asemi-transparent reflector and a backlight at the back side of the LCcell, a transmission type device can be constructed for applicationswith low ambient light levels.

Other objects and attainments together with a fuller understanding ofthe present invention will become apparent and appreciated by referringto the following description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a reflection-type LCD device, according toa first embodiment the present invention;

FIG. 2 is a perspective of the LC device of FIG. 1;

FIG. 3 is a graph of the voltage-versus-reflectivity characteristic (at550 nanometers) for the LC device of FIG. 1;

FIG. 4 is a locus diagram of elliptically-polarized light;

FIGS. 5A, 5B and 5C are graphs of the reflectivity when OFF-vs.-Δndcharacteristic;

FIG. 6A is a graph showing the relationship between the polarizing plateangle and twist angle. FIG. 6B is a graph of the relationship betweenthe twist angle and Δnd;

FIG. 7 is a diagram of a reflection-type LC device in which a polarizingbeam splitter is used as the polarizing element;

FIG. 8 is a cutaway perspective view of a cube section of areflection-type LC device, according to a third embodiment of thepresent invention, that has an active matrix;

FIG. 9 is a cross section of a reflection-type device that is writtenwith light;

FIG. 10 is a cross section of the reflection-type device, according to afifth embodiment of the present invention;

FIG. 11 is a graph of the reflection spectrum when OFF, for the deviceof FIG. 10;

FIG. 12 is a graph depicting the impressed voltage-versus-reflectivity(550 nanometers) characteristic of the device of FIG. 10;

FIG. 13 is a diagram of the change in the LC layer when OFF, for thedevice of FIG. 10;

FIG. 14 is a graph showing the relationship between the twist angleversus Δnd;

FIG. 15 is a cross section of the LC element in the sixth and seventhembodiments of the present invention;

FIG. 16 is a cross section of the LC element in an eighth embodiment ofthe present invention;

FIG. 17 is a diagram of the relationship between the various axes in theLC element; and

FIG. 18 is a graph showing the spectral characteristics when theelectric field is OFF and ON in the LC display element in the sixthembodiment.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS OF THE PRESENT INVENTION

First Embodiment

FIG. 1 is a cross section of a reflection-type LC device, according to afirst embodiment of the present invention, which is comprised of atwisted nematic (TN) liquid crystal (LC) material 104 sandwiched betweena transparent substrate 101 and an opposing substrate 103 having areflector 102. An element 106 polarizes and analyzes any light input andoutput. A transparent electrode 105 puts an electric field on the LClayer. Another electrode is made from a metal thin-film and serves asecond role as reflector 102, which can alternatively comprise areflective layer separated by an insulating layer from a patternedlayer, with the patterned layer at least being transparent. The lightinput, light output, and transparent electrode surfaces are coated witha treatment that reduces and suppresses undesirable light reflections,such as, ghosting, or double imaging.

FIG. 2 illustrates the relative element orientations related to the LCdevice of FIG. 1. An incident (input) light 206, is converted tolinearly-polarized light by a polarizing element. Light 206 is incidentat a polarizing plate angle 205 between an LC molecule director 203 onthe light input side and an electric field surface 204. A plurality ofLC molecules 202 align themselves parallel to the substrate interfacewhen the applied voltage is zero. Processing to properly orient a twistangle 201 between the upper and lower substrates comprisesunidirectional rubbing, oblique evaporation, etc. (See, E. Kaneko,Liquid Crystal TV Displays, KTK Scientific Publishers Tokyo!, c. 1987,pp. 17-18.) The important device parameters for the LC cell are thetwist angle 201 of the nematic LC layer, retardation Δnd, and thepolarizing plate angle 205. (Where Δn is the anisotropy of therefractive index of the LC material, and d is the spacing betweensubstrates.) The direction of rotation, as viewed from the front of thereflector, is the same and positive for all angles below.

FIG. 3 shows the relationship between the voltage and the reflectivityfor the device of FIG. 1. The device has a twist angle 301 of 63°,Δnd=0.2, a polarizing plate angle of zero, a twist angle 302 of 180°,Δnd=0.56, and a polarizing plate angle of 83°. The wavelength used is550 nanometers. (All angles and measurements are approximate.)

FIG. 4 helps explain a change in polarization of light within the LClayer. In a simplest case, the twist angle is 63°, Δnd=0.2, and thepolarizing plate angle is zero. When a linearly-polarized light 402enters and the voltage is zero, the locus of elliptically-polarizedlight will be rotated as shown. It becomes a nearly circularly-polarizedlight 401 at the reflective surface, after which it changes to anelliptically-polarized light 403, whose plane of polarization has beenrotated 90° at the light output surface. In the OFF state, light isblocked at the polarizing plate, and the device reflectivity is impeded.When a voltage is applied, the LC molecules reorient in the direction ofthe electric field, due to the inherent dielectric constant anisotropyof the LC material. In this state, light will not double refract (bebirefringent) in the LC material. Incident, linearly-polarized lightkeeps its polarization as it is reflected and travels back out. Thelocus of elliptically-polarized light changes to circularly-polarizedlight at the reflective surface, and to linearly-polarized light at thelight output surface (which has been rotated 90° from the time of lightinput). The opto-electric characteristics of such a device differ fromprior art modes in that, as shown in FIG. 3, the steepness ofvoltage-transmissivity characteristic curve can be controlled by thetwist angle. This is the same effect caused by the elasticity of twistednematic LC materials (e.g., STN LC displays). The optimum value for thepolarizing plate angle appears in 90° periods. The effect of Δn actsexactly the same as when linearly-polarized light is input perpendicularto the direction of the LC material. Only certain conditions can producethis unique change in polarization. Two optical characteristics of theLC layer are required. First, the layer must change linearly-polarizedlight to circularly-polarized light (with respect to its light input) atthe reflective surface after transmission. Second, the plane ofpolarization of the light must be rotated 90° as it passes through theLC layer. LC cell parameters, below, satisfy these two conditions.

FIGS. 5A, 5B, and 5C show the relationship of Δnd versus reflectivity,when the above device is OFF, and where the twist angle is set and thepolarizing plate angle is zero. The reflectivity will nearly be zerowhen the twist angle is approximately 60° and Δnd=0.2. A twist angle of63° is optimal. The reflectivity when the device is ON, however, isdetermined by the transmissivity of the polarizing element and is nearlyconstant.

FIG. 6A shows the relationship between polarizing plate angles andvarious twist angles. FIG. 6B shows a relationship between the twistangle and Δnd. The wavelength is 550 nanometers for both. The optimumvalues for the polarizing plate angle, Δnd and twist angle values are asabove, and such that continuous state settings were possible. FIGS. 6Aand 6B show the optimum conditions for monochromatic light, but indetermining the actual parameters, it is necessary to trade optimumconditions to get the range of wavelengths that are to be used. Aneffective Δn is necessary, due to the pre-tilt angle of the LC. A twistangle range of 200° is assumed, but state settings exceeding this rangeare possible. A relatively large Δnd is required in this case, thechange in reflectivity due to the wavelength being large, and the usablewavelength range is therefore limited. A small Δnd is needed in order tohold down a change in reflectivity caused by the wavelength chosen, butsince the LC thickness becomes too thin at extremely small values ofΔnd, it must be set somewhere in between these Δnd values. Acceptable LCthicknesses for transmission-type device production can be a problem inreflective-type devices, where the light must travel through the LCmaterial twice (once in, once out). Therefore, Δnd should be slightlylarger than necessary. This increases the production margins possiblefor the elements. When observed at the Δnd=0.2 state above, d changes to2.5 μm (for a typical value of Δn=0.08 in an LC with a small Δn). Incontrast, prior art types with a 45° twist, the optimum LC thickness isless than 2 μm, which results in reduced production uniformity andyield.

The above overall electrode types not intended to display pictures canbe used, e.g., as electrically controlled glare-proof mirrors forautomobiles or as optical shutters in cameras. Particularly when appliedto electrically controlled glare-proof mirrors, a higher reflectivity isobserved when transparent than in two-color pigment types or TN typeswith a polarizing plate in front and back. The threshold characteristicscan be used to increase the number of multiplexed display drive lineswhen used in regular reflective type LC display devices that addressusing an XY matrix.

Second Embodiment

FIG. 7 illustrates a reflection-type LC device, according to a secondembodiment of the present invention, in which a polarizing beam splitter701 is used as a polarizing element. The splitter 701 linearly polarizesa light 703 and directs it toward an LC panel 702. The operation of LCpanel 702 itself is similar to that described above in the firstembodiment. A means for analyzing (polarizing) the output light shiftsit 90° from that of the light input. No light is reflected absent anelectric field, and the voltage-versus-reflectivity characteristicchanges to one that is symmetrical, with respect to the vertical axis(see FIG. 3).

Third Embodiment

FIG. 8 is a cutaway perspective view of a three dimensional section of areflection-type LC device, according to a third embodiment of thepresent invention, having an active matrix comprising a transistor 801positioned at each picture element, an element electrode 802, aninterlayer insulator layer 803, an LC layer 804, a transparent electrode805 deposited on an opposing transparent substrate 806, and a polarizingplate 807. (For a discussion of similar devices, see, NikkeiElectronics, Feb. 16, 1981, p. 164.) This embodiment can also beimplemented in active matrix devices in which TFT, diodes, etc., arearrayed.

Reflection-type displays allow the wiring and active elements of adevice to be positioned under each picture element electrode. A higherpicture element electrode to area ratio can be realized, regardless ofthe required number active elements or amount of wiring. This prevents adecrease in the active viewing area which usually results with a highernumber of picture elements. The advantages achieved are a reduction inthe light loss, compared to guest-host types, the display is brighter,and color images can be produced at lower light powers using colorfilters, since a polarizing plate and diffusion type reflector are notrequired on the bottom (as is typical in prior art TN type reflector LCelements). The retention volume of a thin LC layer is very muchimproved.

Fourth Embodiment

FIG. 9 is a cross section of a reflection-type device, according to afourth embodiment of the present invention. The impedance of points on aphotoconductive layer 901 are changed by light exposure and thatindirectly controls the pattern of an electric field that is applied toan LC layer 902. The device has a reflective mirror 904 and atransparent electrode 904. Similar devices appear in Japanese Laid-OpenPatent Publication 56-43781 and in J. Opt. Soc. Am., Vol. 70, No. 3,p.287 (1980). In the present invention, Δnd=0.2 and the twist angle isapproximately 60°. In the prior art, Δnd is smaller, at 0.18, and the LClayer is usually less than 2 μm. Here, the LC layer thickness can beincreased to be more than 2 μm. The maximum reflectivity in the OFFstate when using a polarizing beam splitter is approximately 80%. Thepresent invention achieves reflectivities approaching 100% in the ONstate.

Fifth Embodiment

FIG. 10 is a cross section of a reflective-type LC device, according toa fifth embodiment of the present invention. It has a twisted nematic LCmaterial layer 1004 that is sandwiched in between a transparentsubstrate 1001 and an opposing substrate 1003 on which is mounted areflector 1002. A transparent electrode 1005 impresses an electric fieldon the LC layer 1004. Another electrode serves also doubles as thereflector 1002, and is a metal thin-film. The light input and lightoutput surfaces are treated with a reflection-reducing coating tosuppress spurious reflections. A circular polarizing plate 1006 has aphase plate 1008 attached to a linear polarizing plate 1007. In thispolarizing element, a quarter wavelength plate is adhered to the LC sideso that the azimuth of the index of refraction is shifted by 45°, withrespect to the axis of transmission of the polarizing plate.Circularly-polarized light is then output. The azimuth of the index ofrefraction of the quarter wavelength plate can be perpendicular to thedirector of the LC. By arranging the principal optical axis on the sideof LC layer 1004 facing quarter wavelength phase plate 1008, and theazimuth of the high index of refraction of the phase plate so they areperpendicular to each other, the advanced and late phases cancel eachother out, thus compensating the reflection spectrum as indicated inFIG. 11, thereby enhancing the brightness of the device.

FIG. 11 has a first reflection spectrum 1101 for the above device whenit is OFF (given, Δnd=0.57 and the twist angle is 180°). A secondreflection spectrum 1102 (for linearly-polarized light incident underthe same LC conditions) is about the same as spectrum 1101, except thatthe polarizing plate angle is set to 83°. This behavior is opposite thatof the first embodiment, with respect to reflectivity on the verticalaxis. But when the device is OFF, the reflection spectrum is wider, inspite of the twist type.

FIG. 12 is a voltage-versus-reflectivity (at 550 nanometers)characteristic for the device of FIG. 10. As in the first embodiment,the steepness of the voltage-transmissivity characteristic curve can becontrolled by changing the twist angle.

FIG. 13 shows a change in polarization in the LC layer when the abovedevice is OFF. The simplest case is one in which the twist angle is 63°,Δnd=0.2 and the polarization angle is zero. Assuming that incident,circularly-polarized light 1302 enters the LC layer via the polarizingelement when the voltage is zero. (Circularly-polarized light which isrotated to the right is assumed here.) The locus of theelliptically-polarized light changes, as shown, to a nearlylinearly-polarized light 1301 at the reflective surface, and is thenreflected back. As the light returns through the LC layer, it convertsto circularly-polarized light 1303 (now rotated in the oppositedirection, leftwise), and is provided as output. Since the direction oftravel of the light has been reversed, the output light, which haspassed through the quarter wavelength plate, converts tolinearly-polarized light, and is able to pass through the linearpolarizing plate. As a result, the device is clear in the OFF state.

When a voltage is applied, the molecules in the LC layer reorient in thedirection of the electric field (due to the dielectric constantanisotropy of the LC material). Any birefringent anisotropy of theincident light vanishes, and incident, circularly-polarized light(rotated to the right) keeps its polarization as it is reflected backout. In the OFF state, the circularly-polarized light is converted tolinearly-polarized light which is perpendicular to the quarterwavelength plate, and is blocked by the polarizing plate, thus reducingthe reflectivity (ON state). This embodiment is opposite in itsfunctioning to the first embodiment, with respect to the reflectivityabout the vertical axis. Such changes in polarization are produced onlyunder certain conditions, e.g., those depicted in FIG. 14, and aresimilar to FIG. 6B of the first embodiment.

Two optical characteristics required of the LC layer are thatcircularly-polarized light become linearly-polarized light with respectto its light input at the reflective surface after transmission and thatcircularly-polarized light be output that has the opposite rotationafter transmission back through the LC layer. The circularly-polarizedlight and the linearly-polarized light have an orthogonal relationshipto one another. Like the first embodiment, the present embodiment can beused as an optical shutter, because of its high reflectivity in thetransparent state. It can also be used effectively in multiplexeddisplay drive of XY matrix LCDs with many lines, by taking advantage ofthe steepness of the voltage-transmissivity characteristic curve whichcan be controlled via the twist angle. The reflection spectrum when thedevice is OFF has a characteristic opposite the first embodiment withrespect to the reflectivity axis, but due to the compensation effect ofthe spectrum, a wider spectrum can be achieved. Further, since there isno specification regarding the polarizing plate angle, there is no needto match the angle in production.

Sixth Embodiment

FIG. 15 is a cross section of an LC device, according to a sixthembodiment of the present invention, comprising LC cell 1501, apolarizing plate 1502, a reflector 1504, an optically anisotropicmaterial 1505, an upper substrate 1511, a lower substrate 1512, atransparent electrode 1513, and LC material 1515. The LC material usedis preferably XLI-4506 (Δn=0.1438) from the Merck Company, and istwisted in LC cell 1501. The cell gap is 5.6 μm and the Δnd is 0.81 μm.The optically anisotropic material is a uniaxially oriented film, madefrom a polycarbonate resin. It has an Δn of 0.0039, a film thickness of80 μm, and Δnd is 0.31 μm.

FIG. 16 is discussed below, relative to the eighth embodiment.

FIG. 17 shows the relationship of each of the axes of the LC displayelements, as seen from above in FIG. 15. A direction 1701 is the axis ofpolarization of polarizing plate 1502, a direction 1702 is theorientation of a uniaxially oriented film 1505 used for the opticallyanisotropic material, a direction 1703 is the rubbing direction of upperplate 1511, and a direction 1704 is the rubbing direction of lower plate1512, angle 1705 is the angle formed between directions 1701 and 1703and is equal to 40° (left), angle 1706 is formed between directions 1702and 1703 and is 22° (left), and twist angle 1707 is 260° (left).

FIG. 18 shows the spectral characteristics of an LC display made underthe above conditions. Spectral characteristics 1801 and 1802 correspondto when the electric field is OFF and ON. The luminous reflectivityY_(off) when the device is OFF is about 83%, and the display color isvery close to being pure white. The luminous reflectivity when thedevice is ON is about 2.0%. Therefore the contrast ratio is 1:42,maximum. A contrast ratio of 1:20 is possible, since the LC displayelement has a large twist angle of 260° and the steepness of thevoltage-transmissivity characteristic curve is extremely good, even whenmultiplexed display drive is being used with duty ratios as severe as1/480.

Seventh Embodiment

Reference is again made to FIG. 15. An LC display element, according toa seventh embodiment of the present invention, is similar to the sixthembodiment, except that LC cell 1501 uses Merck Company LC ZLI-4428(Δn=0.1222). The cell gap is preferably 6.0 μm and the value of Δnd isabout 0.73 μm. A uniaxially oriented film made from a PMMA resin is usedfor an optically anisotropic material 1505. The Δn is 0.00061, filmthickness 600 μm, and Δnd is 0.37 μm.

In FIG. 17, angle 1705 is set at 21° (left), angle 1706 is set at 10°(left), and twist angle 1707 is set at 240° (left). A relatively highluminous reflectivity Y_(off) of 78% is obtained when the device is OFF.The display color is very close to being pure white. The luminousreflectivity when the device is ON is about 2.1%, and so the maximumattainable contrast ratio is approximately 1:37. A uniaxially orientedfilm made from a PMMA resin with an optically negative orientation isused in this embodiment as the optically anisotropic material. Thevisual angle characteristics are excellent as a result.

Eighth Embodiment

FIG. 16 is a cross section of a LC display element, according to aneighth embodiment of the present invention, comprising an LC cell 1601,a polarizing plate 1602, an optically anisotropic material 1605, anupper substrate 1611, a lower substrate 1612, a transparent electrode1613, a reflective film 1614, which also serves as a picture elementelectrode, and a liquid crystal (LC) material 1615. The LC material 1615can be ZLI-4427 (having Δn=0.1127) from the Merck Company. The averageΔnd is 0.69 μm. Reflective film 1614 has a surface roughness 1617.Optically anisotropic material 1605 is preferably a uniaxially orientedfilm made from a polypropylene resin, for which the Δn is 0.0018, thefilm thickness 200 μm, and Δnd is 0.36. An angle 1705 (FIG. 17) valuefor this embodiment is 13° (left), an angle 1706 value is 88° (left) anda twist angle value is 225° (left). A relatively high luminousreflectivity Y_(off) of 80% is obtained when the device is OFF. Thedisplay color is very close to being pure white. The reflectivity tolight when the display is ON is a relatively low 2.2%. This yields arespectably high contrast ratio of 1:36, maximum.

Reflective film 1614 is preferably an aluminum thin-film which has beendeposited by a sputtering method onto the surface of ground glass. Thesurface of the thin-film has a polish of about 0.5 μm, and it is able toreflect light with low sensitivity to directionality. Other metals, suchas nickel, chrome, etc. that have a silver-white color, can be used withacceptable results. The surface can be smoothed with a metal or chemicaltreatment.

The reflective film can be patterned into combs or other figures by adirect method, or indirectly where a transparent electrode on the metalthin-film has an insulator between it and the transparent electrode tobe patterned. Such an insulator has a desirable secondary effect ofsmoothing over the surface roughness, and that can be a big asset whenthe twist angle is very large and the d/p margin (d=cell gap,p=spontaneous pitch) is very narrow.

By positioning a reflector inside the LC cell, as above, the prior artproblem of double images is eliminated. And, minute variations in the LCthickness have the beneficial effect of equalizing the display color andfurther decreasing any discoloration.

In the above embodiments, the optically anisotropic material ispositioned between the LC cell and the polarizing plate, but it can alsobe positioned between the LC cell and the reflector. And, the opticallyanisotropic material is not limited to being only in one layer. Adisplay with a higher contrast ratio and less coloration can be realizedby using it two or more layers.

Wider viewing angles result when rubbing directions 1703 and 1704 aresuch that direction 1702 (of the uniaxially oriented film) is parallelto the horizontal surface of the display. The wider viewing angles makeit convenient for several people at once to view a screen.Unfortunately, the contrast ratio is increased when direction 1702 isperpendicular to the display surface. So a tradeoff must be made. Buteither way, less expensive films that can be used, which substantiallylowers production costs.

By using a semi-transparent reflector 1614 and a backlight on the sideof the reflector opposite the LC cell, a transmission type display canbe made for applications where there is not much ambient light.Reflection-type displays cannot be used under such low light conditions.To make tile transmission type display, a polarizing plate and, ifnecessary, an optically anisotropic material are put between thereflector and the backlight.

Several embodiments of the present invention are described above, butthe present invention is not limited to these, and can be widely appliedto reflection-type optical control devices.

While the present invention has been described in conjunction withseveral specific embodiments, it is evident to those skilled in the artthat many further alternatives, modifications and variations will beapparent in light of the foregoing description. Thus, the presentinvention described herein is intended to embrace all such alternatives,modifications, applications and variations as may fall within the spiritand scope of the appended claims.

What is claimed is:
 1. A reflection-type liquid crystal devicecomprising:a liquid crystal cell including a pair of spaced apartopposed . .transparent.!. substrates, a polarizing means disposed at afirst side said liquid crystal cell, a reflecting means disposed at asecond side of said liquid crystal cell, said liquid crystal cellcomprising a twisted nematic liquid crystal layer having a twist angleand having a birefringence property, Δn, and having a thickness, d, fortransforming the character of first linearly polarized light or firstcircularly polarized light propagating therethrough, respectively, intosubstantially circularly polarized light or substantially linearlypolarized light incident at said reflecting means and transforming saidsubstantially circularly polarized light or substantially linearlypolarized light reflected from said reflecting means, respectively, intosubstantially second linearly polarized light or substantially secondcircularly polarized light having a polarization direction orthogonalwith respect to a polarization direction of said first linearlypolarized light or first circularly polarized light, respectively, saidpolarizing means characterized by having a polarization angle greaterthan zero wherein said polarization angle is an angle formed between apolarization axis of said polarizer and liquid crystal moleculardirector of said liquid crystal layer at a surface of said liquidcrystal cell at said first side, and first and second optimizedconditions for said twist angle relative to a given polarization angleand a product of Δnd, the first optimized condition comprising a twistangle range between about 0° and 65° with a Δnd in a range between about0.13 and 0.40, the second optimized condition comprising a twist anglerange between about 0° and 200° with Δnd in a range between about 0.53and 0.97, said optimized conditions applicable for said givenpolarization angle plus or minus multiplies of approximately 90°.
 2. Thereflection-type liquid crystal device of claim 1 wherein light passingthrough said polarizing means prior to entering said liquid crystallayer is substantially first linearly polarized light and is convertedby birefringence property of said liquid crystal layer to substantiallycircularly polarized light at said reflecting means, and saidsubstantially circularly polarized light is reflected and is convertedby birefringence property of said liquid crystal layer to substantiallysecond linearly polarized light upon passing again through said liquidcrystal layer, said first and second linearly polarized light havingrespective axes of polarization that are substantially orthogonal withone another.
 3. The reflection-type liquid crystal device of claim 2wherein said conversions transpire during an OFF state of said device.4. The reflection-type liquid crystal device of claim 1 wherein lightpassing through said polarizing means prior to entering said liquidcrystal layer is substantially first circularly polarized light and isconverted by birefringence property of said liquid crystal layer tosubstantially linearly polarized light at said reflecting means, andsaid substantially linearly polarized light is reflected and convertedby birefringence property of said liquid crystal layer to substantiallysecond circularly polarized light upon passing again through said liquidcrystal layer, said first and second circularly polarized light havingopposite directions of rotation.
 5. The reflection-type liquid crystaldevice of claim 4 wherein said conversions transpire during an OFF stateof said device.
 6. The reflection-type liquid crystal device of claim 4further comprising a quarter wavelength plate having an optical axis inthe plane of said plate and disposed between said polarizing means andsaid liquid crystal cell, said phase plate and polarizing means forproducing said entering circularly polarized light, said liquid crystalmolecular director substantially perpendicular to the optical axis ofsaid phase plate.
 7. The reflection-type liquid crystal device of claim6 wherein said phase plate has an optical axis shifted by 45° withrespect to an axis of light transmission of said polarizing means. 8.The reflection-type liquid crystal device of claim 6 wherein said phaseplate is a quarter wavelength plate.
 9. The reflection-type liquidcrystal device of claim 1 further comprising:an optically anisotropicelement disposed between said liquid crystal cell and said polarizingmeans, said optically anisotropic element having an optical axis in theplane of said element, said liquid crystal molecular director of saidliquid crystal layer and the optical axis of said optically anisotropicelement arranged relative to one another to provide substantially whitelight during an OFF state of operation of said device.
 10. Thereflection-type liquid crystal device of claim 9 wherein an axis of saidliquid crystal molecular director at said surface of said liquid crystallayer and the optical axis of said optically anisotropic element aresubstantially perpendicular.
 11. The reflection-type liquid crystaldevice of claim 9 wherein second polarizing means and a second opticallyanisotropic element are disposed between said reflecting means and saidliquid crystal cell, said reflecting means comprising a partiallytransparent reflector.
 12. The reflection-type liquid crystal device ofclaim 9 wherein a second optically anisotropic element is disposedbetween said liquid crystal layer and said reflecting means.
 13. Thereflection-type liquid crystal device of claim 9 wherein said opticallyanisotropic element comprises a polycarbonate resin film, polypropyleneresin film or a quarter wavelength plate.
 14. The reflection-type liquidcrystal device of claim 9 wherein said reflecting means has a roughenedsurface to provide light scattering.
 15. The reflection-type liquidcrystal device of claim 14 wherein said reflecting means is disposed onan inner surface of said substrate at said liquid crystal cell at saidsecond side.